This invention relates generally to a reinforcement filler for thermoplastic composite compositions, and more particularly concerns a discontinuous lignocellulose fiber filler.
The intent of filled, reinforced thermoplastic composite technology is to create new materials and market applications by lowering the cost or improving the physical properties of thermoplastics. The cost and performance of the thermoplastic composites are generally a fiction of three variables: (1) the cost and performance of the composite materials, (2) the performance of the resultant composite matrix, and (3) the performance of the interfacial bond between the filler material and the thermoplastic material.
The art of incorporating discontinuous cellulose fiber or discontinuous lignocellulose fiber as a filler in thermoplastic resins to create moldable compositions is well known. Such compositions are known to yield moldable composite compositions with improved tensile strength and flexural properties.
Unfortunately, the physical properties of discontinuous cellulose fiber or discontinuous lignocellulose fiber filler have not, as yet, been addressed as a significant factor relative to the properties of the resultant thermoplastic composite. In fact, conventional fiber-filled thermoplastic composite compositions are relatively indiscriminate as to the source of the fiber, deriving the fiber filler from wood flour, wood chips, rice hulls, used paper, pulp, cellulose powder and mixtures thereof. Moreover, where a wood fiber source is used, the selection of softwood or hardwood to achieve desired end product performance properties is either indiscriminate or unnecessarily specific.
There is some evidence that the physical properties of the fiber filler and resultant thermoplastic composite vary as the source of fiber varies. For example, it is known that the Modulus of Rupture (MOR), a measure of composite brittleness, of discontinuous lignocellulose fiber thermoplastic composites is primarily a function of the source and nature of the discontinuous lignocellulose fiber. Specifically, using discontinuous lignocellulose fiber derived from chemically unaltered (hereafter referred to as "virgin") wood leads to thermoplastic composites possessing significantly higher MOR properties than thermoplastic composites filled with fiber from non-virgin or non-wood cellulose sources.
Further, discontinuous cellulose and lignocellulose fibers commonly used in thermoplastic composites are fine fibers, typically referred to as "wood flour" or "dust". However, longer discontinuous lignocellulose fibers have the capacity to withstand greater stress, and thus have greater tensile properties than shorter fibers of a similar nature. Under load, tensile stress transferred from the composite matrix to the fiber increases from zero at the end of fiber to a maximum value at the fiber's center. As the fiber length increases, the surface area of the fiber increases thereby increasing the distribution of applied stress. As the distributed loading of stress increases over the greater surface area of a longer discontinuous lignocellulose fiber, the amount of stress at a given load at the center of the fiber decreases. Consequently, a longer fiber can absorb greater stress prior to failure than a shorter fiber.
The performance of discontinuous lignocellulose fiber thermoplastic composites is also a function of the concentration of discontinuous lignocellulose fibers in the composite composition. For example, as the pulp wood fiber content in a polypropylene-fiber thermoplastic composite is increased, the tensile and flexural properties of the composition improve until a concentration of 50 percent pulp wood fiber by weight of the composite is reached. Beyond the 50 percent pulp wood fiber loading rate, the tensile and flexural properties of the composite declines.
Related to the concentration of discontinuous lignocellulose fibers in thermoplastic composite compositions is the volume of the fiber in the composite. Fiber volume is a function of the size of the discontinuous lignocellulose fiber and the density of the fiber. Fiber density is determined by the density of the tree species selected as the source of the fiber. At a given length and density, a fiber with a greater diameter will weigh more than a small diameter fiber of similar nature proportionate to the change in the fiber's surface area. However, as the weight of the individual fibers increases, the number of fibers at a given concentration in a thermoplastic composite composition decreases. This decrease of discontinuous lignocellulose fibers within the discontinuous lignocellulose fiber thermoplastic matrix reduces the number of discontinuous lignocellulose fiber and thermoplastic interfaces, which has the same effect as lowering the concentration of the discontinuous lignocellulose fibers in the composite resulting in a composite with reduced tensile and flexural performance.
Coupling agents are usually needed to improve the interfacial bond between the wood fibers and thermoplastic. The coupling agent effectively creates a bridge between the fibers and the thermoplastic which improves the tensile and flexural properties of the thermoplastic composite under load. However, the use of coupling agents adds to the cost of manufacturing the composite composition.
For the foregoing reasons, there is a need for an improved discontinuous lignocellulose fiber filler for thermoplastic composite compositions. The discontinuous lignocellulose fiber should include a relatively high percentage of long fibers to create a large surface area for interface between the fiber and the thermoplastic. The concentration of the new discontinuous lignocellulose fiber in the composite composition should yield increased tensile strength and flexural properties in the composite composition. The tensile and flexural properties of the composite product should improve as the percentage of long fibers are increased. Ideally, the source of the composite materials and manufacturing the discontinuous lignocellulose fiber and the composite composition are simple and cost effective.